Shenzhen Metro Dake Section No.2 well is located in the nursery of Shenzhen Municipal Government on the west side of Tongxin Road, and the cross passage intersects with the tunnel, with a mileage of right SK3 739. The clearance plane in the shaft is 5 m ×6 m in size and 25.724 m in depth. The well passes through plain fill, saturated water-bearing sand layer, soft plastic ~ fluid plastic sandy cohesive soil, completely weathered granite and other strata, and the buried depth of water level is 1.93 ~ 5.73 m, and the shaft is constructed by open-cut covering and composite lining structure.
2 construction methods and implementation points
2. 1 Design and construction of rotary jet water-stop curtain
The high-pressure jet grouting pile waterproof curtain can cut off the seepage of groundwater and prevent sand gushing in foundation pit excavation. The diameter of the pile is related to the soil quality and construction method, which needs to be determined according to the test and engineering practice. Considering the uncertain factors of construction, this project decided to adopt double water-stop piles with a diameter of 60 cm.
The dosage of cement slurry is calculated according to the following formula:
Q = 0125 π d2hα (1β) Q-hardener dosage (m3); D—— design steel bar diameter (m); H—— design pile length (m); α mixing coefficient, α = 1. 1 ~ 1.3, is related to steel bar diameter and soil quality; β operating loss coefficient.
The design diameter of jet grouting pile is D = 60 cm, and the calculated work loss coefficient β is 10%, so the dosage of cement slurry per unit length of jet grouting pile is 340 ~ 400 L/m.
There are double rows of water-rich strata in the wellhead section.
2.2 Shaft construction
2.2. 1 wellhead locking section construction
The wellhead section of the well is 3 m deep and buried in plain fill layer. The maximum excavation plane size of foundation pit at wellhead is 8.6 m× 9.6 m, and C 15 concrete retaining dam is set at wellhead, which is 30 cm higher than the surface. The lock section of the shaft is C25 reinforced concrete. After the pile head of the wellhead section 1 m jet grouting pile is chiseled, it will be excavated by WY60 backhoe, with a depth of 3 m below the surface and a periphery of 30 cm. After the concealed inspection of foundation pit is completed, the structural reinforcement shall be bound and rolled, and various embedded parts between pipes, lifting frames and ladders shall be set. After the formwork is supported, the whole C25 concrete is poured.
2.2.2 Well bore excavation and initial support
The plane excavation size of the standard section of the shaft is 6.4 m ×7.4 m, and it is excavated downwards with the help of a pneumatic pick. The drilling footage for each cycle is a steel grid spacing (0.5~0.75 m), and a sump is set in the middle. Firstly, the sewage pit is constructed, and then it is extended to the shaft wall for trimming and modeling. After excavation and molding, C20 concrete with a thickness of 5 cm is sprayed, and then the net is hung, with L = 3.0 m.
After the shaft excavation exceeds the depth of the jet grouting pile waterproof curtain (below the surface 17.5 m), sand and mud gush in the sandy cohesive soil layer due to the rich groundwater. In order to ensure safety, the original design was changed according to the geological conditions during construction.
2.3 Wharf Door Construction
The place where the shaft enters the cross passage, commonly known as "Matoumen", is the weak point of construction and excavation. Because the well in Matoumen area is soft plastic sandy cohesive soil, the original design vault single row was changed according to the geological conditions during construction.
2.4 Secondary lining of shaft
After the excavation and initial support are completed, the secondary lining of the bottom hole should be completed first. After the initial support of Matoumen was closed, the secondary lining structure of the shaft was constructed in sections from bottom to top, and the secondary lining construction of the remaining shafts was carried out in three times from bottom to top, and closed with the first grouting lock section. The pouring methods are central fixed formwork support reinforcement, double pump pouring, C25 commercial concrete construction and internal vibrator vibrating. The second lining of the transverse passage in Matoumen section and the second lining of the shaft in this section are poured integrally to improve the structural stress integrity, and the rest standard sections of the shaft are poured twice.
2.5 shaft hoisting
The shaft is equipped with two 100 kN electric hoists, and a bucket of 2.25m3 is hoisted as vertical transportation equipment for excavation and blanking. During the construction of the second lining of the interval tunnel, a 30 kN electric hoist is added between the walking beams of two sets of 100 kN electric hoists, which is used for the second lining reinforcement to go down the well. The main structure of the lifting frame consists of six columns, two longitudinal beams, three walking beams and five cross beams. The column is welded with the steel plate embedded in the foundation, and the column, longitudinal beam, walking beam and cross beam are connected by M20 bolts.
3 Conclusion
When the shaft passes through the complex stratum, it should follow the construction principles of "advanced pipe, strict grouting, short footage, strong support, early closure and diligent measurement", and it is absolutely not allowed to rush blindly. In addition, for the cavity caused by local collapse, grouting pipe should be reserved under the condition of strengthening the initial support, and grouting should be carried out behind the initial support to make it dense and prevent the collapse from expanding. In the construction, advance geological prediction should be made, such as using Luoyang shovel to probe holes and geological radar to detect; It is necessary to strengthen construction management and establish unified command to ensure construction safety.
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